Automatic frequency control



Oct. 9, 1951 R A, BRADEN 2,570,758

I AUTOMATIC FREQUENCY CONTROL V V Y V V Aff. m man/feb* A ATTORNEY Oct. 9, 1951 R. A. BRADEN AUTOMATIC FREQUENCY CONTROL 3 Sheets-Sheei 2 Filed Sept. 4, 1946 INVENTOR 733126 ./Zrcn BY f ATTORNEY Oct. 9, 1951 R. A. BRADEN 2,570,758

AUTOMATIC FREQUENCY CONTROL ATTORN EY Patented Oct. 9, 1951 AUTOMATIC FREQUENCY coN'rRoL Rene A. Braden, Lawrence Township', Mercer County, N. J., assignor to'Radio Corporation of America, a corporation of Delaware Application September 4, 1946, Serial No. 694,753

15 Claims. (Cl, Z50-36) 'This invention relates to the frequency control of high frequency systems. An object of the present invention is to maintain a constant frequency difference Vbetween the output of a local oscillator and a source of in- .ccming waves. Y

Other objects will appear from a reading of the following description which is accompanied by :drawings wherein:

" Figs. 1 through 6 show six different embodiments of the invention as applied to a signal receiving system.

' Although the invention is described in connection with a signal receiving system, it should be understood that the invention is not limitedy vthereto but has general application to other vsystems wherein a frequency control is required. Throughout the figures of the drawing, the 'same parts are represented by the same reference numerals. p

' 'Referring to Fig. 1 of the drawing, there is vshown a signal receiving system comprising an antenna having a reflector I and in the focus thereof, a dipole II. rective structure for receiving waves transmitted thereto froma remote source. It should be understood that this antenna is merely illustrative of any suitable wave collecting structure. The outputof the antenna is coupled to -the 'input of a suitable radio frequency sellective stage I2. The output of stage I2 is coupled to a frequency converter or mixer tube I3 which is supplied with oscillations from a Alocal heterodyning oscillator I4 via line I5. The output from the mixer I3 is "a beat frequency which is the sum or differ-' rence of theV frequencies of the incoming wave and the oscillations produced by the local heterodyning oscillator I4. This beat frequency is an intermediate frequency which is supplied to the. intermediate frequency amplifier stage 24. The' `output of the intermediate frequency stage 24 /is fed via one output tube to a suitable utilizaition circuit over lead I6, and also via another output tube over leads I1 to the automatic fre-. r 'quency control circuit constituting the gist of 'the present invention. The two output tubes in ythe I, F. amplifier stage 24 serve to separate the two loads in order to prevent detuning of the discriminator or change of its selectivity and atv` .the same time prevent the discriminator from" A interfering with the operation of the utilization circuit.

The automatic frequency control system of This antenna is a wave di- D. C. amplifier I9, a suitable power supply 2l and connections which couple these circuits to each other and to the local oscillator I4 in such manner as to maintain a constant frequency difference between the frequency of the incoming waves and the frequency of the local oscillator, The local oscillator I4 is shown as a reflex Klystron The oscillator may generate oscillation's of the order of 4,000 megacycles per second although this frequency is given merely by way of'illustration. `Oscillator I4 is shown as comprising an evacuated envelope having a cathode K for producing a beam of electrons, a toroidal cavity resonator M of suitable dimensions and having apertures in both sides to permit electrons fromfthe cathode K to pass therethrough, and a reflector or repeller electrode R. The voltage on the repeller electrode is of negative polarity relative to the cathode, and repels the electrons passing through the cavity resonator M. The electrons in this Klystron oscillator move to and fro through the cavity resonator M until they are finally collected. The dimensions of the cavity resonator M and the voltage on the reiiector electrode R determine to alarge extent, the frequency of the oscillations generated by the reflex Klystron I4. Such an 'oscillator is well known in the art and no claim is made to it per se.

Output from the local oscillator I4 is taken from a coupling loop L located in the interior of the cavity resonator M. This loop is coupled to a coaxial transmission line I5 which extends to one of the electrodes of the mixer or frequency converter I3.

The negative voltage on the reflector electrode R of the local oscillator I4 is supplied by means of frequency departure fromY the desired constant frequency difference (beat frequency applied to .discriminator I8), and has a magnitude which depends Vupon the extent of this frequency departure from the desired constant frequency difference.

The D. C. amplier I9 comprises a pair of Fig. lincludes a frequency discriminator I8, a 55 vacuumltubes A and i3 arranged in series, as

3 shown and a resistor network including a gain control resistor or potentiometer P. Output is taken from this gain control resistor P via a tap or slider associated with lead 2|. Maximum gain is obtained when the tap on resistor P is nearest the anode of tube B. This D. C. amplifier is shown and described. in detail in lll.,S.`- Patent 2,310,342, granted February 9,A 1 943, to M. Artzt to which reference is 'hereinfmade This D. C. amplifier circuit I9 not only compensates for frequency changes of either the local oscillator I4 or the source of incoming waves collected on the antenna II), II, that is, compensates for any departure from a predetermined beat frequency or frequency difference between them,` butalso compensates for voltage Variations in the iioating power supply 20 and in-the powersupply Which supplies the anode polarizing l'potential for the cavity resonator M of the local oscillator Via lead 23.

The frequency discriminator [B comprisesV a pair of diode rectiers D and D'l arranged backto-back. The anodes of these 'diodes are coupled to opposite terminals of a parallel'tuned circuit T, and this tuned circuit 'I' is magnetically coupled to another tuned fcircuit T. It should be noted that the electrical center of the inductance coil of the tuned circuit T is capacitively coupled to the high potential end of thev tuned circuit T', and furthermore that this electrical center point is coupled via a choke coil C tothe electrical center of a resistor-shunt capacitor arrangement connected'between the cathodes of the diodes. The output of the intermediate frequency amplifier 24 which comprises' the'b'eat frequency, is connected va leads I'I to the 'tuned circuit T.V The output of the discrir'ninator" I8 is connected between the grid and the cathode of the tube B of the D. C. amplifier I9.

In the operation ofthe discriminator I8', the output of thiscircuit, asv applied across the grid and cathode of tube IB of the D. C. amplifier, is

zero or a minimum whenthebeat lfrequency in leads I'I applied to'the'"di'scriminator`is'a'predetermined frequency equal to the desired Yconstant frequency difference between the output of the local oscillator I4` an'd the frequencyy of the incoming waves. If this beat frequencyin lead I'I should vary. to one sidefor` the other of the desired constant frequency difference due to a change in frequency of the Vlocal oscillator I4 or achange in frequency ofthe incomingwaves collected on the antenna, thefvoltagleappearing in the output'of the discriminator willalso Vary. one side or the other of the minimum or zero" voltage. Putting Yit in other words, the polarity of the voltage inthe output of discriminator- I8 depends upon the direction of the frequency departure from the desired'constantfrequency difference and the magnitude of this. voltagein thev Y voltage on the gain'control resistor yP andthis-f` variable voltage is passed through thel floating bias or power supply 20 to the reilector electrode R. The variation in voltage appearing 'o n; reflector electrode R of the local oscillatoris ofV Asuch sense and magnitudeas to compensate for V quency control of Fig. l.

argon lamps'.

frequency changes or departures from the desired constant beat frequency in leads II.

In summation, it will be evident that the voltage developed in the output of the discriminator I8 is a function of the frequency difference between the heterodyning frequency (frequency of .the local oscillator I4)l yand, the frequency of the incoming signal" anddthat this varying voltage is' applied to the reflectorY electrode of the local oscillator I4 by way of a D. C. amplifier I9 and the power supply 2U.

'Fig 2 is a modification of the automatic freyOnly those circuits have been shown in Fig. 2 which are necessary 'to an understanding of the invention. In Fig. 2,

the cathode K of the local oscillator I4 is supplied with ayariable voltage from the D. C. amplifier I9 while the repeller electrode R is maintained at a constant potential with respect to ground.- It should be noted that the bias for the repeller electrode R is obtained by virtue of a tapon resistor S which is y-part ofthe resistor n'etworl:l of the D. C. amplifier I 9;' The powersupply" 20 in Fig. 2 is so connected that its positivejjsermnal supplies voltage to the cavity resonator M while its negative terminal is connected to the cathode K,

Fig. 3 is ra `modification ofjthe system-oifig. 2 and is somewhat similar inroperationthereto except for the use of a 'cathodeffollowe'r vacdum tube F between the cathode YK of the localoscillator I4. and the gain control 'resistor 'Pofc'the D. cA amplifier ze. .This cathode-'follower a1- lows the gain of the D. C;jarnpliiier I9- tobe higher than ypermissible in the system of Fig. 2, because in' Fig. 2 the local Oscillator 'I4 isafload on the D. C. amplifier and reducesithe eiet'z'tive gain, whereas in r'ig. 3 the grid of the' cathode follower has a very high'impedanceand isfnot as great a load on the D. C. amplifier.vv vItjshggilld be noted that in Fig. 3, the" need for "a 'floating power supply is eliminated.

Fig. 4 isa further modification which is preferred over the arrangement of Fig. 3. In Eig. 4, the anode voltage or' ,vo l'tage for the cavity'resonator M is taken from a tap on a separate. resistor W- rather than directly fromthe D. C. amplifier as shown in yFig. 3'. vThe arrangement of Fig. Yl `avoi'dsar'iy possible unbalanceingthe D. C; amplifier circuit. It fshould be. .'-.no.tcd further, that, the/voltagey for therepeller'l?. of the local oscillator Isis now'obtained'frmare- .sistor 3@ forming part of a.voltagestabilizerfcircuit employinga number of.; glow' lamps or voltage regulatortubes 3l and 32 arranged in series.

These voltage regulator tubes'llmmaylbe fneon. or Thel number of lamps inY series depends inthev voltagerequired. 'Thevoltage drop across lamps 3l and32 Vis constant ife'nough current is appliedtofcauselthem toV light. `.This constant voltage drop is 'applied to resistorA 30, and a selected'fraction thereoffis applieditd the repeller electrode R.V Itfshoul'd be notejdthat the discriminator I8 is atground.potentiahland that only one power supplyisfrequired. Thfoutput fromv the discriminatori is. relatively small but this output is ampliiiedbyV the D. C. amplifier Fig. 5 is 'a refinement over the circuito'f Fig'. 4 and differs.from.'V Fig. 4f'in the use of `a second set of voltage' regulator tubes or' glow lamps 3I' and 32 across the cavity'resonator and cathode ofthe local oscillatorv I4pso as to maintain .the

voltage` of the' anodeor. cavity" resonator M 'constant vrelative to the cathode Here again as other.

in Fig. 4, the repeller electrode is held at a constant voltage by glow lampsL 3I and 32. The voltage on the grid of the cathode follower F which varies in accordance with the variation in amplified A. F. C. voltage on the gain control resistor P of the D. C. amplifier I9 changes the potential of the cathode K or" the local oscillator I4 with respect to its repeller electrode R, thus controlling the frequency of the local oscillator I4. Obviously, the tap on resistor 30 which is connected toA the repeller electrode R is so adjusted that the local oscillator I4 oscillates. Suitable time constants are introduced in the discriminator or between it and the D. C. amplifier grid. In order to operate the system of Fig. 5, approximate voltages relative to ground at points |00, IGI and |02 may be +200 volts, +500 volts and +50 volts respectively.

Fig. 6 illustrates a still further modification which eliminates the use of a D. C. amplifier shown in the other figures. It should be noted that the cathode K of the local oscillator I4 is directly connected to the anode of the cathode follower tube F, and that there are provided a pair of voltage regulator tubes 3|" and 32 across the anode and cathode of tube F. The discriminator I8 has one output terminal connected to the grid of the follower F and the other output terminal connected through an adjustable bias battery G to the common negative terminal of the circuit, which may be also a ground connection. A regulated voltage supply appears in lead 33 for use on the anode of local oscillator I4 and on the voltage regulator tubes. Power for the local oscillator is provided by a power supply whose negative side is grounded. This voltage is regulated by a Voltage regulator. The output voltage of the regulator is divided between the .resistor 50 and the regulator lamps 3l", 32

Which are so proportioned that the drop acrossV resistor 50 will be the correct anode-to-cathode voltage for the oscillator and the drop across the tubes 3l", 32 will be comewhat greater than the required cathode-to-repeller electrode voltage. The voltage applied to the cathode follower is stabilized by the voltage regulator 3l", 32, while the total voltage across resistor 50 and tubes 3|", 32 is held constant by the Vacuum tube voltage regulator. Hence, the anode voltage ris also stabilized. The bias battery G is so adjusted that the local oscillator I4 oscillates at the center of its range when no controlling signal is impressed on one of the intermediate frequency output tubes 24. Putting it in other words, over a certain range of adjustment of the bias battery the oscillator Ytube will oscillate, since this bias acting through the cathode follower controls the repeller voltage. The bias battery should be set at the center of this range, as this allows the A. F. C. Voltage to effect control over equal ranges in each direction from the value obtained when the control voltage is zero. adjustment is preferred in all the circuits, though the required condition is arrived at in different ways in the different circuits. Obviously, if the manual adjustment is left at the point where the tube just started to oscillate, the A. F. C. would work when frequency deviated inone direction from the desired value, but notin the In making initial adjustments, one sets the voltages so the tube oscillates, checks frequency, then tunes the vcavity so that the tube oscillates at the desired frequency. Generally several successive readjustments of cavity and repeller voltage must be` made .to get. the frequency x at the right value with Ofv course,;a similar the reseller voltage at the middle of the range within which it causes the tube to oscillate, i I

What is claimed is:

1. An automatic frequency control system comprising a local vacuum tube oscillator having electrodes and having a frequency of oscillation which is responsive to the voltage difference therebetween, a source of waves with which the frequency of said oscillator is to be compared, and means for maintaining a constant frequency difference between the frequency of oscillation of said oscillator and the frequency of said source, said means including apparatus for beating said waves with the oscillations from said oscillator to derive beat frequency oscillations, a frequency discriminator having a pair of coupled tuned circuits, connections to supply said beat frequency oscillations to said discriminator, a balanced direct current amplifier in the output of said discriminator, and circuit connections from said amplier to two electrodes of said oscillator to control the voltage therebetween.

2. An automatic frequency control system comprising a local vacuum tube oscillator having electrodes and having a frequency of oscillation which is responsive to the voltage difference therebetween, a source of waves with which the frequency of said oscillator is to be compared, and means for maintaining a constant frequency difference between the frequency of oscillation of said oscillator and the frequency of said source, said means including apparatus for beating said waves with theoscillations from said oscillator to derive beat frequency oscillations, a frequency discriminator having a pair of coupled tuned circuits, connections for supplying said beat frequency oscillations to said discriminator, a balanced direct current amplifier in the output of said discriminator, and circuit connections from said amplifier to two electrodes of said oscillator to control the voltage therebetween, one of said last connections including a floating bias supply for one of said two electrodes of said oscillator.

3. An automatic frequency control system comprising a local vacuum tube oscillator having electrodes and having a frequency of oscillation responsive to the voltage on one of said electrodes,

a source of waves with which the frequency of` said oscillator is to be compared, and means for maintaining a constant frequency difference between the frequency of oscillation of said oscillator and the frequency of said source, said means including apparatus for beating said waves with the oscillations from said oscillator to derive beat frequency oscillations, a frequency discriminator having a pair of coupled tuned circuits, connections for supplying said beat frequency oscillations to said discriminator, a balanced direct current amplifier in the output of said discriminator, said amplifier having an adjustable gain control resistor, and a connection from said gain control resistor to said one electrode of said oscillator.

4. An automatic frequency control system comprising a local vacuum tube oscillator having electrodes and having a frequency of oscillation responsive to the voltage on one of said electrodes, a source of waves with which the frequency of said oscillator is to be compared, and means for maintaining a constant frequency difference between the frequency of oscillation of said oscillator and the frequency of said source, said means including apparatus for beating said waves with the oscillations from said oscillator to derive beat frequency oscillations,

f,ai.frequencyidiscriminatori' haring azpairs of .cou-

.gplediitunedncircuitsn:connectionstyfor supplying said beat frequency oscillations.to.;..s.aid;-discrim inator, a balanced directzcurrentiamplifier in 4tial relativefito said. oscillator --cathode-,I and a .asthefcutputcf.saiqcdiscriminatornsaidampuer 5- connection.from.` the repellcr electrodertofsaid -:.w;haying an adjustable gain; controlxelenienu-fand ,zamlolifleryaconnection, including agcathode.ijfollower.tube, 8. All.A :automatic frequency contro1;.:"SySt.em from saidgain.controllelementto .saiclxtone elec- Ycomprising a reflex oscillator having elf-cathode', .trede ofsaidcsciilator. a cavityfresonator and a recoller electrode, a 5.. An; `automatic .sfrequencyiifcontroli.-systernv 1o Sourceof :Waves with Which the frequeney 0f .comprising aglocalVacuum.tubesnscillatorzihavs.; saidv oscillator..A iS to bel compared,V and*A means .f ing electrodes.ana-having a-irequencyiofi-oscil- -x.for-maintaining a constant frequency difference @..lationi-responsive.,to the Vvvoltage;cnfsaidr.e1ec- '.betweenthefrequenoy of said oscillatorr/andithe f.;trodes, a1source.: ofv waves with lWhich; the. fre- A 'frequency-.of said sou1Ce,. Said lneansxncluding ...squencyofisaidoscillator is. to be;icompar.ed,..and- 15..-al3paratue '.-f01`.=beatng .Sad WaVeS WithftheflOS- .=;-meansffor. maintaining aI/.constantfrequencydif- 4eiiiations:fromr-Sad` oscillator to `loroducenioeat ference between the i-frequency ofgoscillation of ifrequencyoscillations, a frequelflcy` discrimina- .saidgoscillatorandethefrrequencyorjsaid source, tor, connections for supplying saidwzbeatffresaidmeans including apparatus .forbeatingsaid fluency loscii'latons tosaid-discriminatori, a bal- -fwwayesgwithy-the oscillationsfromsaid; oscillator` 20 anceci: direct current amplifiersV in -thaoutnut' of to;derive beat frequencyoscillations, a frequency Saidi ,discrimination a. gain control resistor.; in K iscrimnator, @(jyygectigns foy;4 Supplying 'said Sadmpllflel, a CELhOde OHOWSI 'hVI'IgagIJ-.d beat frequency oscillations to saidr discriminaconnected to.:said;;control resistor and alcath- .5013 a5 balanced directA @umani-,.ampler in the ode connected tothe cathode" of saidoscilla'tor, Output 0f Said disrminator, Said amplier hau/ 4 25 a direct .current powersupply connectedftoimaining anadjustabiegain Control, ,elementL and a, tainsaid cavitycresonator. at a-.positive'potenconnectionL including a cathode follower. tube, tial relative to..said oscillator-cathode, andfin- -vfrcrrlv Said gain Coni-,rol e'lglneni-L:50 onewof Said dvidllai vllltfafge;ifStiilllzllig CIYCLltSf. OIISad electrodes ofgsaid oscillator; :and a v.voltage stacavityresonator and isaid repeller.-electrodeyione ;..-bi1izing; circuit for. maintainingat least one 30 saidstabilznacirout to .maintain saifl` resonaother .electrode Ofsaid.. oscillator at a.. constant .tor at a. Substantially *iiXedi/.Oltage Withresneet yoltage witlrresloectv to a commongroundY conto said cathode: andthe; otherwsaid .stabilizing ,netion'ofthe SystgmY t0 said .amplier, over .circuit .to maintainsaid repeller.electrodeI at Wide rang@ @fem-rents, a. substantiallyrxed voltage with .respect .to a f. 6....Ang; automatic.; frequency .control system. 35 :eolnlnon'ground @Onneeton between Sadreloel- .zcomprising areiexcscillator having a cathode, `er'e1ect1odei`-and5aidialnpleri a cavity.. resonator and a repeller electrode, a 9. `n automatic frequency control Ysystem Vsource oft/aves with'. whichv theY frequency of .comprising aV reflex'oscillatorhavng aecathode, said..oscillator is tobe compared, and. Ameans a Cavity resonatorV and a-repelier electrode, a for .maintaining aconstant `frequency diiference 40 .scurce'of Waves Withwhicl'i the frequencyfof-S'aid .1, between the frequency. of oscillation of said ososcillator is to.v be compared, and means: -for `cillatorand the frequency of saidiyvaves, .said maintaining a Constant frequency difference bemeans. including apparatus for beating` said tween the frequency of said oscillator andthe Waves. with the oscillations frornsaid. oscillator frequency of said source, Said means including .to produce; beat frequency. oscillations, a fre-1 45 aDDaratuS fOr-beatinglsai-d Waves with the 0S- quency discriminator, Connections. for supplying cillationsfrom. said oscillator t0 produce beat .the beat-frequency oscillations to. said discrimifrequencyL oscillations, a frequencyl discrimina- Y nator, a balanced direct current v'amplifier-in the 'tory eonneetion'for Supplying Said beat frequency g output. of said discriminatori, a gain control. .reoscillations to: said discriminator, abalanced disistor in said amp-liner, a cathode foilowerhavto rect current amplifier in the output Vof `saldfolis- .ing a grid connected to said control resistor .and .erininaton a gainv control resistor in said .arna cathode connected to the cathode ofsaidrcsplier, a cathode: followerhaving a grid Vconcillatcr., adirect current powerisupply..Connected .nested to lsaid control resistor anda cathode g. to maintainsaid .cavity resonatorat a positive l connected to the cathode of said oscillator,said ,potential .relative` to said oscillatorrcatnode, and 55 Aainpler lhaving tWo :Vacuum tubes-serially cona voltagestabilizing circuit forinantaining .said nected and having a resistor networkconnected cavityresonator .potential substantially constant Y.across said tubes, and. individual connections relative to the oscillator cathode. from the cavtyfresonator and the repeller elecf 7. An. automatic frequency control system trode to different ...points on .said resistor'netcomprising a renex. osciliatornaving a cathode, oo work havngfdlrerent. .direct potentials.

a cavity resonator and a repeller electrode, a 10. An automatic frequency control VSystem source of Waves with Which the frequency: of comprising av local Vacuum tube oscillator. having said oscillator is to .be compared.. and means electrodes andhaving a frequency of-oscillation for maintaining a .constant frequency. difference, responsive to thevoltage difference therebetween, between the frequency of said oscillator and the 05 a sourceofzwaves `with which the frequency' of frequency of said source, said meanszincluding .'saidoscillator is to be compared, and means'for apparatus for heating'. said waves rwith .the osy maintaining a constant frequency diiferencefbecillations from said oscillator. to produce4 beat tween the'frequency of oscillation of said. oscilfrequency oscillations, a frequency discriminalator and the frequency of said source, said means tor, connections for supplying. saidr beat fre- 70 including'apparatus for vbeating said lwavesfwith quency oscillations to said discriminator,V a bal- .the oscillations fromzsaid oscillator to derive: beat .-,anceddirect current arnpliiier` in. the output of frequency oscillations, a frequency discriminator, vfisaid discriminator, a gain control resistor insaid connections to supply said ybeat frequency-oscilamplier, a .cathode follower having afgridconvlations to said discriminator, and a cathode fola..nected .to saidcontrol'resistor and a. cathode 'ldowerhaving agridconnected tosaid discriminmeans including apparatus for beating said waves with the oscillations from said oscillator to derive beat frequency oscillations, a frequency discriminator, connections to supply said beat frequency oscillations to said discriminator, a balanced direct current amplifier in the output of said discriminator, a cathode follower having a grid connected to receive the output of said amplifier and having a cathode and an anode connected to said oscillator electrodes to control the voltage therebetween.

12. An automatic frequency control system comprising a local vacuum tube oscillator having electrodes and having a frequency of oscillation responsive to the voltage difference therebetween, a source of waves with which the frequency of said oscillator is to be compared, and means for maintaining a constant frequency difference between the frequency of oscillation of said oscillator and the frequency of said source, said means including apparatus for beating said waves with the oscillations from said oscillator to derive beat frequency oscillations, a frequency discriminator, connections to supply said beat frequency oscillations to said discriminator, and a cathode follower having a grid connected to said discriminator to receive the output thereof and a cathode and an anode connected between said oscillator1 electrodes to control the voltage therebetween, and a voltage stabilizer circuit coupled to the cathodes of said follower and of said oscillator.

13. An automatic frequency control system comprising a local vacuum tube oscillator of the klystron type having an ungrounded cavity resonator with repeller and cathode electrodes on opposite sides of the resonator, said oscillator having a frequency of oscillation which depends upon the voltage difference between said electrodes, a source of waves with which the frequency of said oscillator is to be compared, and means for maintaining a constant frequency diiference between the frequency of oscillation of said oscillator and the frequency of said source, said means including apparatus for beating said waves with the oscillations from said oscillator to derive beat frequency oscillations, a frequency discriminator, connections for supplying said beat frequency oscillations to said discriminator, a balanced direct current amplifier in the output of said discriminator, said amplifier having an adjustable gain control element, a cathode follower tube having a grid connected to said gain control element and having a cathode connected to the cathode of said oscillator, and a voltage stabilizing circuit for maintaining the repeller electrode of said oscillator at a constant potential with respect to ground over a wide range of currents.

14. An automatic frequency control system in accordance with claim 13, characterized in this, that said voltage stabilizer circuit comprises a pair of serially arranged regulator tubes connected between ground and the positive terminal of a source of unidirectional potential through a resistor, another resistor in shunt to said pair of regulator tubes, and a connection from said repeller electrode to a tap on said last resistor.

15. An automatic frequency control system comprising a local vacuum tube oscillator of the klystron type having an ungrounded cavity resonator with repeller and cathode electrodes on opposite sides of the resonator, said oscillator having a frequency of oscillation which depends upon the voltage difference between said electrodes, a source of waves with which the frequency of said oscillator is to be compared, and means for maintaining a constant frequency difference between the frequency of oscillation of said oscillator and the frequency of said source, said means including apparatus for beating said Waves with the oscillations from said oscillator to derive beat frequency oscillations, a frequency discriminator, connections to supply said beat frequency oscillations to said discriminator, and a cathode follower having a grid coupled to said discriminator and having a cathode coupled to the kcathode of said oscillator.

RENE A. BRADEN.

REFERENCES CITED The following references are of record in the file of this patent:

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